It is often important to be able to determine the ratio of fluids in a multicomponent multiphase fluid system. For example, in the petroleum industry, fluid produced from a hydrocarbon formation commonly includes oil, water and gas. It is important to know the net oil production from a well and the volume of gas and water in the produced fluid, as this information is utilized in a variety of areas, such as, for example, in the determination of royalty payments, productivity measurements, the cost of lifting production fluid, equipment sizing and reservoir and well management.
A number of different ways of measuring oil, gas and water production rates have been suggested. A commonly used method has been to simply introduce a predetermined volume of production fluid into a three-phase separator vessel where the water settles beneath the oil and the gas rises above it, after which the individual phases are measured. This is not an entirely satisfactory method, however, because it is slow, requiring many hours and frequently days for the emulsions of oil, water and gas produced by wells to separate in the vessels. It may also be necessary to add expensive chemicals to enhance this separation. In addition, the apparatus is required to be semi-continuously operated, with data being acquired by visual and manual means, manually recorded and subsequently utilized in carrying out suitable mathematical calculations in order to obtain the information sought. The apparatus is necessarily large, expensive and cumbersome and is such that satisfactory operation requires great care and skill on the part of the technicians operating it. Also, use of such apparatus and its related method of testing production fluid for net oil content has frequently resulted in an error of plus or minus 10%, which is not acceptable by today's standards.
Other suggested methods make use of a variety of measurements and procedures to determine the flow rate and composition of multiphase fluid mixtures. There is, however, no inexpensive, reliable commercially available instrumentation for measuring oil, water and gas production rates from a producing oil well, and in particular, for accurately measuring the production rates of individual components of a multiphase fluid comprised of very large amounts of water or gas on a real time basis. While this discussion has been primarily in connection with measurement problems relating to multiphase fluid produced from a hydrocarbon formation, it is noted that similar measurement problems exist in connection with other three-component multiphase fluid systems.
It is an object of the present invention to provide a method for determining the composition of fluids in a three-component multiphase fluid system which is accurate over the entire range of the components from 0% to 100%, uses commonly understood thermodynamic and instrumentation technology, requires no chemicals for phase separation, lends itself to automation, is not space or capital intensive and provides accurate results. More specifically in connection with the petroleum industry, it is an object of the invention to provide such a method which is applicable to the determination of the composition of water, gas and oil in a multiphase system comprised of these fluids. The ability to use such a method in the field in a remote location would be of further benefit.